When a wireless device powers on or moves between cells in a wireless network, the wireless device receives and synchronizes to downlink signals in a cell search procedure. The cell search procedure identifies a preferable cell and performs time and frequency synchronization to the network in downlink (e.g., from a base station to a user equipment).
A user equipment (UE) may use primary and secondary synchronization signals (PSS and SSS), such as those described in Section 6.11 of Third Generation Partnership Project (3GPP) TS 36.211, version 11.2.0, for performing a cell search procedure, such as the cell search procedure described in Section 4.1 of 3GPP TS 36.213, version 12.1.0.
FIG. 1 illustrates an example radio frame and locations of OFDM symbols used for PSS and SSS, according to a 3GPP specification. The X-axis represents orthogonal frequency division multiplexing (OFDM) symbols 112 and the Y-axis represents subcarriers 114. Each resource element 113 corresponds to one OFDM subcarrier 114 during one OFDM symbol interval 112. Particular OFDM symbols 112 include PSS 116 and SSS 118.
3GPP specifies that for frequency division duplex (FDD) PSS 116 is mapped to the central 6 resource blocks (RBs) (using the central 72 subcarriers) in the last symbol of time slots 0 and 10 within a radio frame and the SSS is transmitted in the OFDM symbol preceding PSS, such as illustrated in FIG. 1.
3GPP specifies that for time division duplex (TDD) PSS is mapped to 6 RBs (using the central 72 subcarriers) in the third symbol of time slot 2 (subframe 1) and the third symbol of time slot 12 (subframe 6) within a radio frame and the SSS is transmitted in the last OFDM symbol of slots 1 and 11 (i.e., three symbols ahead of the PSS).
The SSS and PSS have the same frequency mapping for both FDD and TDD, but in different OFDM symbols. In the frequency domain, 3GPP specifies that the PSS and SSS are transmitted in the central subcarriers irrespective of the system bandwidth, which allows the UE to synchronize to the network without prior knowledge of the allocated system bandwidth. The synchronization sequences use 62 subcarriers in total (with 31 subcarriers mapped on each side of DC subcarrier) extended with five zeroes at the edge and mapped to the center 73 subcarriers. The DC subcarrier is not used for transmitting signals.
FIG. 2 illustrates an example of an initial cell search procedure. A UE typically may have a frequency error of 2 to 20 ppm (part per million) at power on. This corresponds to 40 to 400 kHz frequency error at a carrier frequency of 2 GHz. The UE then tries to detect a PSS. From the detected PSS, the UE may derive the cell id within a cell-identity group, which consists of three different cell identities corresponding to three different PSS. To perform the detection, the UE searches for all of the three possible cell identities. The UE may also determine an OFDM symbol synchronization and a coarse frequency offset estimation with an accuracy of about 1 kHz. The UE estimates the latter by evaluating several hypotheses of the frequency error.
The UE then detects the SSS. From the detected SSS, the UE acquires the physical cell id and achieves radio frame synchronization. The UE also detects whether the cyclic prefix length is normal or extended. A UE that is not preconfigured for a particular duplex mode (e.g., TDD or FDD) may detect the duplex mode by the frame position of the detected SSS in relation to the detected PSS. The UE may estimate fine frequency offset by correlating PSS and SSS. Alternatively, the UE may use cell-specific reference signals (CRS) to estimate fine frequency offset.
After synchronizing with the PSS and the SSS, the UE may receive and decode cell system information, which contains cell configuration parameters such as the Physical Broadcast Channel (PBCH). The number of OFDM symbols used for PDCCH (Physical Downlink Control Channel) is signaled by PCFICH (Physical Control Format Indicator Channel) according to Section 6.7 of 3GPP TS 36.211, version 11.2.0. The PCFICH is decoded before the UE receives PDCCH. The number of OFDM symbols signaled by PCFICH may be 1, 2 or 3 for large bandwidth allocations (e.g., more than 10 resource blocks) and 2, 3 or 4 OFDM symbols for small bandwidths (e.g., less than or equal to 10 resource blocks). The first OFDM symbols of a subframe are used for PDCCH.
Section 6.10.1 of 3GPP TS 36.211, version 11.2.0, illustrates CRS mappings for one, two, and four antenna ports. As illustrated in the 3GPP specification, CRS are not mapped on the same OFDM symbols as used for PSS and SSS.